The background will be described with reference to prior art publications listed in the attached appendix.
Magnetic resonance (MR) imaging is the most accurate non-invasive test for assessing tears of the menisci and the cruciate ligaments of the knee (1,2). MR imaging of the knee has replaced conventional arthrography in evaluating meniscal and ligamentous disorders and has decreased the morbidity and the cost associated with negative arthroscopy (3,4). Additionally, much progress has been made in recent years in imaging articular cartilage (5,6). However, there is still a lot of disagreement on which pulse sequences are best suited for imaging articular cartilage. Current sequences are often limited by insufficient spatial resolution and inadequate signal-to-noise ratio (SNR) (7).
In magnetic resonance (MR) imaging, the scan sequence type and scan parameters are chosen prior to the scan. The choice of sequence and parameters depends on the desired image signal-to-noise ratio (SNR) and contrast-to-noise (CNR). The signal level and contrast can be determined from a signal equation for the sequence which relates signal level to the scan parameters and the tissue parameters (T1, T2 and proton density).
The primary goal of cartilage imaging is to accurately depict cartilage structure and abnormalities, which requires high resolution and high SNR. The tissue characteristics of cartilage, specifically its short T2 and low proton density, make this challenging. The amount of joint fluid is often increased with patients with cartilage damage or other articular disorders. In this instance, joint fluid can fill cartilage and tears as well as meniscal tears. Thus a high image contrast-to-noise ratio (CNR) between joint fluid and other tissue can be very useful for diagnoses of cartilage or injuries or meniscal tears, as has already been shown clinically using saline MR arthrography (8,9). However, MR arthrography is invasive, so that there is much demand for non-invasive imaging methods. A useful method of imaging cartilage would show joint fluid brightly while also preserving cartilage signal, thus allowing visualization of both the cartilage surface and substance.
There are currently many popular methods used in knee imaging and cartilage imaging. Recht and Resnick (10) provide a good overview of these methods. Many groups (5,11) have found that spoiled gradient recalled echo imaging provides very desirable contrast characteristics. An analytical parameter optimization for gradient recalled echo (GRE) including spoiled gradient-recalled acquisition in the steady state (SPGR) techniques is given in (12). A comparison of 3D SPGR with "standard" methods including T1- and T2-weighted SE, gradient-recalled acquisition in the steady state (GRASS), and 2D SPGR is presented in (13).
Fast spin echo (FSE) imaging has become very popular owing to its high scan-time efficiency. FSE imaging allows for proton density (PD) weighted images and T2-weighted images, both of which can provide good contrast between cartilage and joint fluid (14). Contrast-enhanced techniques are also being used in knee imaging. Gd-DPTA.sup.2 --imaging (15,16) and sodium imaging (17) are methods which image the proteoglycan content in the cartilage, which is thought to be early sign of degenerative cartilage. Other methods which are useful in knee imaging include magnetization transfer contrast (MTC) (6,18), dual-echo in the steady state (DESS) (19), and diffusion-weighted imaging (20).
Driven equilibrium Fourier transform (DEFT) imaging has been used in the past as a method of signal enhancement (21-25). Because this signal enhancement depends on the tissue T1 and T2 it has been discovered in accordance with the present invention that DEFT can be used to generate tissue contrast while maintaining a high signal to noise ratio. Frequently, a choice has to be made between SNR and CNR. By attenuating the signal of one tissue, the CNR can be increased, but the SNR for that tissue drops. However, DEFT imaging tends to simultaneously achieve high SNR and CNR. Some tissues, particularly those with short T2 and long T1 are difficult to image with a high SNR. With DEFT imaging, these tissues are often seen as brightly or more brightly than with other sequences. The DEFT signal enhancement is greatest for tissues in which the T2 is a reasonably high fraction of T1. The end result is that both tissues have a good SNR, and there is strong contrast between the tissues.
In one application, DEFT generates contrast between cartilage and joint fluid by enhancing the signal from joint fluid, rather than by suppressing the cartilage signal like some sequences do.